Cover plate assembly for sealing spaces between turbine buckets



June 16, 1964 w. M. FARRELL 3,137,478

COVER PLATE ASSEMBLY FOR SEALING SPACES BETWEEN TURBINE BUCKETS FiledJuly 11, 1962 INVENTOR WILLIAM M FARRELL, BY m-1 HIS ATTORNEY.

United States Patent 3,137,478 COVER PLATE ASSEMBLY FOR SEALING SPACESBETWEEN TURBINE BUCKETS William M. Farrell, Scotia, N.Y., assignor toGeneral Electric Company, a corporation of New York Filed July 11, 1962,Ser. No. 209,099 7 Claims. (Cl. 253-77) This invention relates to animproved cover plate for sealing oif fluid leakage between certainportions of turbine buckets, and more particularly to an improved meansof attachment therefor.

In some types of turbines operating with very high temperature motivefluid, such as gas turbines, the blades are often attached to theturbine wheel by means of long, relatively thin shanks, which connectthevane of the bucket with the dovetail base portion. These shanks makeit possible to attach the bucket to the turbine wheel at a location oflower temperature. With such constructions, segmented cover plates havegenerally been employed to block oil the gas flow between the narrowbucket shanks and also, in some cases, to provide other functions suchas holding the buckets in place and serving as gas seals with adjacentstationary membersof the turbine. These cover plates may be held inplace at their radially outermost parts, and thereby loaded incompression whenv the wheel rotates, as disclosed in US. Patent3,043,562 issued in the names of F. H. Van Nest and G. R. Fusner on July10, 1962, and assigned to the assignee of the present application.Alternately, the cover plates may be held at their radially innermostends and thereby loaded in tension, as disclosed for example in US.Patent 2,998,959 issued to L. Hayworth et al. on September 5, 1961.

In many cases, the local loading stress on the turbine Wheel caused bythe cover plate is greater than that caused by the turbine bucketitself. This is because the bucket shank and bucket vane are ofrelatively thin crosssection and a plane taken radially through theshank and bucket normal to the turbine axis reveals a relatively smallamount of metal acting upon the dovetail base in that plane. On theother hand, a radial planetaken through the cover plate normal to theturbine axis often reveals the local loading to be quite high in thatplane.

Whether the cover plate is held on the turbine rim by means of hooks orlugs holding the cover plate at its innermost end, or whether the coverplate loads the underside of the bucket platform, the load is ultimatelytransferred to the rim of the turbine wheel. The increased local loadingon the rim is more than often accompanied by bending moments, due to thefact that the cover plate is not radially in line with the point whereit ultimately loads the turbine wheel rim. These bending moments serveto increase the local stress, and make the means of attachment of thecover plates a critical matter.

Accordingly, one object of the present invention is to provide improvedcover plate structure and means of attachment therefor.

cover plate which loads the rim of the turbine wheel without imposingadditional bending moments thereon.

Still another object of the invention is to providean improved coverplate which, in addition to sealing olf flow between buckets, providesthe additional functions of holding the turbine buckets against axialdislodgement from the wheel and serves as a mounting for flanges whichform a circumferential sealing ring.

Yet another object of the invention is an improved means of lockingturbine buckets together at the platforms when the wheel is rotating.

A more specific object of the invention is an improved cover plate,which is attached in a very eflicient manner to permit a low weightpart, and which loads the turbine wheel rim in a uniform manner freefrom bending moments. V

Briefly stated, the invention is practiced by providing a segmentedcover plate, eachsegment having a dovetail base which fits in an axiallylengthened dovetail slot in the rotor rim together with the dovetailbase of the long shank turbine bucket. I

The subject matter which is regarded as the, invention is particularlypointed out and distinctly claimed, in the concluding portion of thespecification. The invention, however, both as to organization andmethod of practice, together with further objects and advantagesthereof, may best be understood by reference to the followingdescription taken in connection with the accompanying drawings in which:

' FIG. 1 is a portion of a turbine wheel, looking in an axial direction,showing several long Ishank turbine buckets, with cover plates, mountedon a turbine wheel FIG. 2 is a cross-section, looking in acircumferential direction along lines II-H of FIG. 1, showing the coverplate and means of attachment to the rim;

FIG. 3 is a plan view in cross-section, taken along lines IIIIII of FIG.1, through the cover plates only.

FIGS. 4 and 5 are enlarged schematic views, looking in an axialdirection, of themeans for locking turbine bucket platforms together,shown when the turbine wheel is atrest and when it is turning,respectively; and,

FIG. 6 is an exploded. perspective View showing the dovetail attachmentfor the improved cover plate.

Referring now to FIG.'1 of the drawing, a portion of a turbine rim 1 isshown which is furnished with a group of circumferentially-spacedaxially extending dovetail slots 2 extending around its periphery.Diposed in each of the slots 2 is a long-shank turbine bucket, showngenerally as 3, which includes a vane portion 4, an-arcuate bucketplatform 5, which forms a portion of the inner boundary wall for themotive fluid flowing through the turbine, and a radially extendingbucket shank 6. Shank 6 serves to connect platform 5 to a dovetail baseportion 7 which fits in slot 2. Throughout this specification and in theclaims, it will be understood that the term dovetail refers to themultiple dovetail or fir treettype of attachent, as the term isgenerally understood in the turbine art. I

Extending radially between rim 1 and bucket platforms 5 are a number ofimproved cover plates, shown generally as 8, which likewise havedovetail base porthe cover plates 8 thus fit in the remaining length ofthe Still another object of the invention is to provide a tions 9fitting'in dovetail slots 2, 'togetherwith the base portion 7 of thebuckets. The cover plates have relatively thin flat plate portions 8a,which serve to block gas flow between shanks 6 and which are taperedtoward their radially innermost ends where they are attached to baseportions 9. Plate portions 8a are preferablyithinner toward theiroutermost edges. The tapered plate portions 8a may be provided witharcuate axially-extending flanges 10, which cooperate to form twocircumferential, axially extending sealing rings. These rings form closeclearances with a stationary diaphragm (not shown) to prevent theflow ofgas radially between th'e'rotor and the diaphragm. Cover plates 8 alsodefine notches 11 at their radially outermost edges which support theopposite ends of locking pins 12.

Referring to FIG. 2 of the drawing, it will be observed that the axiallength of the dovetail slot 2 in the rotor rim is substantially greaterthan the axial length'of the bucket dovetail base portion 7f Thedovetail bases 9 of slot 2 on either side of the bucket bases 7 I orstaked to prevent dislodgment. larly noted that there are no radialhooks or lugs with this means of attachment to increase windage lossesfor the rotor.

Referring to FIG. 3 of the drawing, the manner of sealing, against gasleakage between cover plates 8 will be apparent. The cover plates aredivided into two types: inner cover plates 15 and outer cover plates 16,having half-thickness overlapping edges 15a and 16a respectively.Although there will be no tendency for the outer tips of the coverplates to move away from the buckets, they are nevertheless arranged toprevent this possibility. Inner cover plates 15 are not allowed to moveaxially away from the buckets because of the overlapping edges 16:; ofthe outer cover plates. Hence, the radially outer tips of cover plates15, indicated as 17 in FIGS. 1 and 2, terminate just short ofthe bucketplatforms 5, for ease of insertion. The outer cover plates 16, however,have extending lips seen at 18 in FIGS. 1 and 2, which extend intogrooves a in the bucket platforms so as to prevent lips 18 from movingoutward. Grooves 5a are formed of ample size to allow ease of insertionof the outer cover plates 16, and clearances provided so that the outercover plates 16 do not actually touch or load the bucket platform 5.

Reference to FIGS. 2, 4 and 5 together will indicate the manner ofoperation of locking pin 12. The pin is cylindrical and is notched atits ends to provide semi-circular extensions 12a which fit in notches11in the inner and outer bucket cover plates. When the wheel isstationary, as in FIG. 4, the pins merely rest in grooves 11, but whenthe wheel is rotating, the pins assume the position shown in FIG. 5. Theforce exerted by centrifugal action is very great and serves to centerthe pin between adjacent bucket platforms 5 and effectively lock themtogether so there is no relative movement between buckets. This is not aclamping device, but actually serves to clamp the bucket platformsfirmly together.

The exploded view of FIG. 6 shows the method of assembly of the coverplate into the slot 2. It will be observed that the rim 1 of the turbinewheel is provided with an axial projection 1a and an undercut groove 1b,which correspond to axial projection Sa and undercut groove 9b on thecover plate base 9.

To assemble the cover plate to the turbine wheel, first, the baseportion 7 of the turbine bucket 3 is centered in slot 2 and then thebase portions of the cover plates are inserted from either side of thewheel. Grooves 1b and 915 on the wheel and cover plate respectively arealigned and the retaining strip 13 (see FIGS. 1 and 2) is then insertedradially outward to fit in the aligned grooves, at the same timealigning its holes with those in the cover plate base. Then pins 14are'inserted and secured in place. It will be apparent that strip 13then prevents relative axial movement between rim extensions 1a andcover plate base extensions 9a. Therefore, both buckets and cover platesare locked to the rim 1 and can move neither radially nor axially.

The arrangement and construction of the cover plate 8 is such that itpresents a moment-free'loading on its dovetail base portion 9. Referenceto FIG. 2 of the drawing will show that a radial plane w passing throughthe axial center of the dovetail base portion 9 represents a line alongwhich the base 9 will be loaded in tension with no bending moments. Inother words, if the moments of the cross-sectional area taken from line19 have a sum of zero, then there will be no bending moments imposed ondovetail base 9 to add to its tensile loading.

The metal in the cover plate proper is so distributed and arranged oneither side of line 19 that the bending moments in opposite axialdirections are balanced. Here the bulk of the tapered plate portion 8ais balanced by the extending sealing flanges it which have a muchgreater effect in bending than the plate portion 801, since they extendfarther in an axial direction. The axial ex tension 9a of the baseportion 9 has a negligible effect, since it carries only its own weight.

The operation and advantages of the improved cover plate will beapparent from the following description. A dovetail arrangement isone ofthe most efiicient turbine bucket attachments known, for such a partwhich has to be removable. The theories and design of the contours ofthe dovetail slot and mating dovetail portion to fit in that slot havereached a high degree of perfection. Hence it is that the improved coverplate, by virtue of being retained inplace on the rim by a dovetailbase, permits an extremely light-weight cover plate. The cover plateimposes its own uniform tensile force on the rotor rim without imposingany bending moments thereon. cover plate shown, by virtue ofdistributing the metal about the center of the dovetail base portions 9,exerts a pure radial force on the rim and has no tendency to bend inwardor outward under the action of centrifugal force. The cover plate shownneither loads the turbine bucket (which, in turn, would increase thelocal loading on a portion of the bucket base portion), nor'doesit-attach to the bucket or turbine rim by means of lugs or hooks (whichcreate undesirable bending moments and/ or local increased stresses).

Locking of bucket platforms 5 together is accomplished uniformly acrossthe axial surface of the platform by means of the pins 12, which areconveniently held in place by notches in the cover plates when the Wheelis at rest. Since the cover plates are made in segments, i.e., one foreach turbine bucket, they are easily replaced in the event that rubbingof the seals provided by flanges 10 takes place. The overlapping featureof the cover plates shown in FIG. 3 prevents the leakage of gas .betweenplates. Although the preferred embodiment is with inner and outer coverplates, they can also be manufactured identical in form so that theyoverlap con- Y secutively in shiplap style. The means of attachment ofthe cover plates to the rim with retaining strip 13 and pins 14 reduceswindage losses, and also prevents the axial movement of the buckets inthe slots 2.

While there has been described what is at present considered to be thepreferred embodiment of the invention, it will be understood thatvarious modifications will occure to those skilled in the art, and it isintended to cover in the appended claims all such modifications as fallWithin the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is: l

1. A cover plate for a long shank turbine bucket comprising:

a fiat plate portion tapering in front elevation and havtend radiallyoutward from a turbine rotor rimby said dovetail base portion willcancel. 2. A cover plate for a long shank turbine bucket comprising:

a fiat plate portion tapering in front elevation and including at leastone arcuate flange extending normal to the plate across its width, and adovetail base attached to the smaller radially inner end of said platedefining dovetail grooves extending in the direction of the platethickness, said plate portion The and flangebeing disposed with respectto a plane passing through the center of the base portion transverse tosaid grooves so that bending moments caused by centrifugal forces whenthe plate is attached to extend radially outward from a turbine rotorrim by said dovetail base portion will cancel, whereby the flat plateportion is substantially balanced by the flange.

3. In a turbine, the combination of a turbine wheel having a rimdefining circumferentially spaced, axially extending dovetail slots, aplurality of long shank turbine buckets having dovetail base portionsdisposed in said slots, said bucket base portions having axial lengthsless than those of said dovetail slots, a plurality of cover platesdisposed at opposite sides of the bucket shanks, each plate havingradially extending flat plate portions tapering in front elevation anddisposed adjacent one another so as to block the spaces betweenbucketshanks, each plate portion having a dovetail base portion disposed in adovetail slot adjacent a bucket base portion, and means preventingrelative axial movement between the cover plate base portions and theturbine wheel rim, whereby the cover plates and turbine buckets areindividually supported by their respective dovetail base portions.

4. In a turbine, the combination of a turbine wheel having a rimdefining circumferentially spaced, axially extending dovetail slots, aplurality of turbine buckets having dovetail base portions disposed insaid slots,'the buckets also having arcuate platform portions radiallyspaced from said dovetail base portions and connected thereto byrelatively slender shanks, a plurality of cover plates disposed atopposite sides of the bucket shanks, each plate having a radiallyextending flat plate portion tapering in front elevation and havingopposite radial edges defining flanges overlapping portions of similaradjacent cover plates, each of said cover plates also having attached tothe smaller radially inner end thereof a dovetail base portion disposedin a dovetail slot adjacent a bucket base portion, the flat plateportions of said cover portions and connected thereto by relativelyslender shanks, a plurality of cover plates disposed at opposite sidesof the bucket shanks, each plate including a dovetail base portiondisposed in one of said dovetail slots, there being a cover;platedovetail portion at either side of each bucket base'portion andhavingattached thereto .a fiat plate portion tapering in front elevationand explates being so disposed with respect to the cover plate tendingradially betweensaid rim and said bucket platforms, said plate portionsalso including at leastone axial: ly extending arcuate flangethereon,'the fiat plate portions and flange being so disposed withrespect to the cover plate base portion so as to balance bending momentsabout the base portion caused by centrifugal forces when the wheelrotates to provide moment-free loading of the cover plates on the rim. I

6. The combination, according to claim 5, including a plurality ofcylindrical locking pins extending over the axial length of the bucketplatforms and loosely supported in notches defined at the radially outerportions of axially opposite cover plates, said notches being disposedradially inward from the bucket'platfor'ms where they are'adjacent oneanother, so that said pins will move outward under the action ofcentrifugal force to lock the bucket platforms together. Y

7. The combination, according. to claim 5, wherein said cover plate baseportions and said rim include axially extending projections definingaligned radially undercut grooves, arcuate retaining strips disposed insaid grooves to prevent relative axialmovement between cover plates andturbine rim, and means fastening said retaining strips 7 in saidgrooves, whereby both cover plates and turbine buckets are held insaiddovetail slots against radial and axial disengagement.

ReferemieslCited the file of this patent UNITED STATES PATENTS 2,755,063

' Wilkinson July 17, 1956 2,920,865 1 Lombard Ian; 12, 1960 2,985,426Hunter et al. May 2-3, 1961 2,988,325 Dawson June 13, 1961 2,994,507Keller et al Aug. 1, 1961 2,998,959 Haworth et al. I -Q'Sept. 5, 19612,999,668 Howald et al. "Sept. 12, 1961 3,010,696 Everett Nov. 28, 19613,037,741 1 Tuft June 5, 1962 3,043,562 Van Nest et al JulylO, 1962FOREIGN PATENTS 652,099 Great Britain Apr. 18, 1951

3. IN A TURBINE, THE COMBINATION OF A TURBINE WHEEL HAVING A RIMDEFINING CIRCUMFERENTIALLY SPACED, AXIALLY EXTENDING DOVETAIL SLOTS, APLURALITY OF LONG SHANK TURBINE BUCKETS HAVING DOVETAIL BASE PORTIONSDISPOSED IN SAID SLOTS, SAID BUCKET BASE PORTIONS HAVING AXIAL LENGTHSLESS THAN THOSE OF SAID DOVETAL SLOTS, A PLURALITY OF COVER PLATESDISPOSED AT OPPOSITE SIDES OF THE BUCKET SHANKS, EACH PLATE HAVINGRADIALLY EXTENDING FLAT PLATE PORTIONS TAPERING IN FRONT ELEVATION ANDDISPOSED ADJACENT ONE ANOTHER SO AS TO BLOCK THE SPACES BETWEEN BUCKETSHANKS, EACH PLATE PORTION HAVING A DOVETAIL BASE PORTION DISPOSED IN ADOVETAIL SLOT ADJACENT A BUCKET BASE PORTION, AND MEANS PREVENTINGRELATIVE AXIAL MOVEMENT BETWEEN THE COVER PLATE BASE PORTIONS AND THETURBINE WHEEL RIM, WHEREBY THE COVER PLATES AND TURBINE BUCKETS AREINDIVIDUALLY SUPPORTED BY THEIR RESPECTIVE DOVETAIL BASE PORTIONS.